Chiller systems typically incorporate the standard components of a refrigeration loop to provide chilled water for cooling a designated building space. A typical refrigeration loop includes a compressor to compress refrigerant gas, a condenser to condense the compressed refrigerant to a liquid, and an evaporator that utilizes the liquid refrigerant to cool water. The chilled water can then be piped to the space to be cooled.
Chiller systems that utilize so called centrifugal compressors can typically range in size, for example, from ˜100 to ˜10,000 tons of refrigeration, and can provide certain advantages and efficiencies when used in large installations such as commercial buildings. The reliability of centrifugal chillers can be high, and the maintenance requirements can be low, as centrifugal compression typically involves the purely rotational motion of only a few mechanical parts.
A centrifugal compressor typically has an impeller that can be thought of as a fan with many fan blades. The impeller typically is surrounded by a duct. The refrigerant flow to the impeller can be controlled by variable inlet guide vanes (“IGV”s) located in the duct at the inlet to the impeller. The inlet guide vanes can operate at an angle to the direction of flow and cause the refrigerant flow to swirl just before entering the compressor impeller. The angle of the inlet guide vanes can be variable with respect to the direction of refrigerant flow. As the angle of the inlet guide vanes is varied and the inlet guide vanes open and close, the refrigerant flow to the compressor can be increased or decreased. In many applications, the inlet guide vanes can be variable ninety degrees between a fully closed position perpendicular to the direction of the refrigerant flow to a fully open inlet vane guide position in which the inlet guide vanes are aligned with the refrigerant flow. When the cooling load is high, the inlet guide vanes can be opened to increase the amount of refrigerant drawn through the evaporator, thereby increasing the operational cooling capacity of the chiller.